1. Field of the Invention
This invention generally relates to mobile wireless communication networks such as cellular telephone networks. More specifically, the present invention relates to a system and method for managing assignment of spreading codes used in a mobile cellular or wireless communication system using spread spectrum communication technology. The invention is useful for is systems providing different data rates (e.g., data services and voice users).
2. Description of the Related Art
The following references, incorporated by reference herein in their entireties, are helpful to an understanding of the principles of spread spectrum communications: (1) E. Dahlman and K. Jamal, xe2x80x9cWide-band services in a DS-CDMA Based FPLMTS System,xe2x80x9d IEEE VTC 1996, pp. 1656-1660; (2) ETSI SMG2, xe2x80x9cThe ETSI UMTS Terrestrial Radio Access (UTRA) ITU-RTT Candidate Submission,xe2x80x9d May/June 1998; (3) K. S. Gilhousen, xe2x80x9cSystem and Method For Orthogonal Spread Spectrum Sequence Generation in Variable Data Rate Systems,xe2x80x9d U.S. Pat. No. 5,751,761, May 1998; and (4) Otani, xe2x80x9cSmooth Spectrum Spreading with a Small Spreading Factor,xe2x80x9d U.S. Pat. No. 5,530,717, June, 1996.
Code Division Multiple Access (CDMA), also called Spread Spectrum (SS), is a technique for increasing the number of users or mobile stations that may be accommodated in a wireless communication system, such as a digital cellular phone service. There are three general categories of CDMA: Frequency hopping, direct sequence, and time scrambling. Direct Sequence (DS) CDMA is one of the potential multiple access candidates for an advanced communication system such as the IMT-2000 system which supports high-data-rate services and variable-bit-rate services, such as speech. For example, the variable-bit-rate service of the DS-CDMA based IMT-2000 system is supported by way of multi-code transmission or single-code transmission using a variable spreading factor (VSF). Reference (4) describes a DS-CDMA system.
In reference (1), Dahlman, et al. compared multi-code transmission and single-code transmission using a variable spreading factor to implement variable-bit-rate services in a DS-CDMA based system. In particular, the performance, complexity, and flexibility of the two transmission methods were studied. The study concluded that single-code transmission with variable spreading factors allows for simpler implementation of variable-rate services and is preferred from a complexity point-of-view. On the other hand, multi-code transmission, which uses orthogonal codes for different connections on the forward link (downlink), is preferred from a capacity point-of-view. As a result, Dahlman, et al. recognized benefits of using a hybrid solution wherein multi-code transmission is used on the forward link and single-code transmission with variable spreading factors is used on the reverse link (uplink).
In reference (2), ETSI SMG2 submitted a universal mobile telecommunication system (UMTS) terrestrial radio access (TRA) proposal to ITU as a candidate radio transmission technology (RTT) solution for IMT-2000. Orthogonal variable spreading factor (OVSF) codes are used as channelization (spreading) codes to distinguish between channels. OVSF codes can preserve the orthogonality between channels of different rates using different spreading factors.
Reference (3) proposed a method for allocating a set of orthogonal pseudo-noise (EN) code sequences of variable length among user channels operative at different data rates in a spread spectrum system. Orthogonal Walsh codes of variable length are employed to modulate the information signals. The code assignment is made on the basis of channel data rates in a manner that results in an improved utilization of the available frequency spectrum. In particular, Gilhousen proposed an ASSIGNED list and a BUSY list to handle the code assignment process. However, Gilhousen did not present efficient and optimal methods to handle the code.
Conventional spreading/modulation units in a CDMA system are illustrated in FIGS. 1 and 2, and the generation of OVSF codes is illustrated in FIG. 3.
In a CDMA system, each cell site (base station) has many modulator-demodulator units. Each modulator-demodulator unit at the base station is assigned to a mobile station as needed to provide a communication path between the base station and the mobile station. Examples of spreading/modulation units for downlink and uplink channels are discussed in reference (2) and shown in FIGS. 1 and 2, respectively.
FIG. 1 shows a spreading/modulation unit 10 having a channel 12 constituting a dedicated physical data channel (DPDCH) for carrying data generated at a second layer and a dedicated physical control channel (DPCCH) for carrying control information generated at a first layer. P(t) is a pulse shaping filter (root raised cosine, roll-off 0.22). A total number of bits per DPDCH/DPCCH channel determines a spreading factor (SF) used in the spreading/modulating unit 10. Each spreading factor corresponds to a fixed data rate requested by a user (e.g., different rates for data services or for voice users) and, e.g., may range from 4 to 256 in accordance with 256/2k wherein k ranges from 0 to 6.
FIG. 2 illustrates a spreading/modulating unit 20 having a DPDCH channel 22 and a DPCCH channel 24. Data modulation is based on dual-channel quadrature phase shift keying (QPSK), where the channels DPDCH and DPCCH are mapped to the I and Q channels, respectively. Data in the I and Q channels are spread to the chip rate with two different channelization codes CD and CC, and subsequently complex-scrambled by a mobile-station specific complex scrambling code Cxe2x80x2scramb and optionally another scrambling code Cxe2x80x3scramb. P(t) is a pulse shaping filter (root raised cosine , roll-off 0.22).
OVSF codes as disclosed in reference [2] can be used as channelization codes CD and CC to preserve mutual non-interference among channels of different rates and different spreading factors. In particular, rate sets supported by the OVSF codes can be summarized as follows: a code with SF=i can support a basic rate of 256/i R0 (e.g., a code with SF-256 can support a basic rate R0, a code with SF-128 can support a basic rate 2R0, and a code with SF=64 can support a basic rate 4R0), and so on, noting that a maximum number of basic rate users supported by the system is 256. R0 is a predetermined basic rate defined by the system. A code tree defining the OVSF codes is shown in FIG. 3.
In FIG. 3, each level in the code tree defines channelization codes of length spread factor (i.e., SF=1 in level 1, SF=2 in level 2, and SF=4 in level 3). Not all codes within the code tree can be used simultaneously within one cell. A code can be used in a cell if and only if no other code on the path from the specific code to the root of the tree or in the subtree below the specific code is used in the same cell. This means that the number of available channelization codes is not fixed but depends on the data rate and spread factor of each physical channel.
Random assignment of large-SF codes to low data rate (high spread factor) channels may preclude the use of a large number of small-SF codes, thus inefficiently limiting the number of remaining codes.
Despite the fact that the code is a valuable resource in a CDMA system, none of the references discussed above-discloses an optimal code assigning algorithm to provide an efficient and optimal method for allocating spreading codes, reducing the complexity of the code assignment process, and improving system utilization.
In view of the foregoing disadvantages with the prior art techniques, it is an object of the present invention to provide an improved system and method for managing assignments of spreading codes according to variable data rates requested by users such that a minimum number of spreading codes are assigned by way of an optimal codeword. Users operative at a particular data rate are assigned to closely related codes so as to minimize the number of small-SF codes being marked as unavailable.
The present invention is directed to a code-assignment system and method for managing either multi-code or single-code transmission in, e.g., a wideband CDMA system using OVSF codes. The present invention achieves the above object by establishing a system codeword to represent available spreading codes that can be used in a spread spectrum communication system, determining an optimal codeword based on a user defined weight and the system codeword, and assigning spreading codes based on the optimal codeword.